As positive type photoresist compositions there are normally used compositions comprising an alkali-soluble resin and a naphthoquinonediazide compound as a light-sensitive material. Examples of such compositions include novolak type phenol resin/naphthoquinonediazide-substituted compound as disclosed in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470. Most typical examples of such compositions include novolak resin made of cresol-formaldehyde/trihydroxybenzophenone-1,2-naphthoquinonediazidosulfon ic ester" as disclosed in L. F. Thompson, Introduction to Microlithography No.219, pages 112-121, published by ACS.
As a binder, novolak resins can be dissolved in an alkaline aqueous solutions without swelling. The novolak resin can also exhibit a high resistance particularly to plasma etching when an image thus produced is used as a mask for etching. Thus, novolak resins are particularly useful in this application. As a light-sensitive material, a naphthoquinonediazide compound itself serves as a dissolution inhibitor for reducing the alkali solubility of novolak resins but is peculiar in that it undergoes decomposition upon irradiation with light to produce an alkali-soluble substance which rather enhances the alkali solubility of novolak resin. Because of the great change in properties caused by irradiation with light, the naphthoquinonediazide compound is particularly useful as the light-sensitive material for a positive type photoresist.
From such a perspective, many positive type photoresists comprising novolak resin and naphthoquinonediazide light-sensitive material have heretofore been developed and put to practical use. These positive type photoresists have attained sufficient results in forming lines of a width of as small as 1.5 to 2 .mu.m.
However, integrated circuits have further increased degree of integration. In the production of semiconductor boards such as for SLSI, it has been required to form very fine patterns of 1 .mu.m or less side. In such application high sensitivity photoresists are desired from the viewpoints of particularly high resolution, high precision in reproducing patterns to exactly copying the shape of exposure mask and high productivity.
It is a recent tendency that the etching process is switched from wet etching processe to dry etching process to improve resolution and enhance the degree of integration in integrated circuits. However, since the dry etching process is subjected to a rise in the temperature of the resist, the resist to be used in this process is required to exhibit a high heat resistance to avoid heat deformation.
Examples of approaches for improving the heat resistance of the resist include the use of a resin free of components with a weight-average molecular weight of 2,000 or less as disclosed in JP-A-60-97347 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and the use of resin wherein the total content of monomers, dimers and trimers is 10% by weight or less as disclosed in JP-A-60-189739.
However, the use of the above mentioned resin free of or having a reduced amount of low molecular weight components is disadvantageous in that it normally causes a reduction in sensitivity, lowering the throughput in the production of devices.
It has been attempted to improve the sensitivity or developability of a resist composition by incorporating specified compounds to the resist composition. For example, U.S. Pat. No. 4,738,915 discloses a positive type photoresist composition containing trihydroxybenzophenone. The use of such a positive type photoresist composition containing trihydroxybenzophenone enables improvements in sensitivity and developability but is disadvantageous in that the incorporation of trihydroxybenzophenone causes deterioration in the heat resistance of the composition.
In approaches as disclosed in JP-A-64-44439, JP-A-1-177032, JP-A-1-280748, and JP-A-2-10350, an aromatic polyhydroxy compound other than trihydroxybenzophenone is used to provide a higher sensitivity without deteriorating the heat resistance. However, these approaches leave to be desired in the improvement in developability.